When materials burn or explode, emission of light in the visible as well as the invisible infrared (IR) and ultraviolet (UV) wavelengths occur. The chemical composition of the burning flame determines the wavelengths of light emitted. These emissions can be detected by various photosensitive devices for safety, process control or spectroscopic purposes.
Fire detection systems which furnish an electrical output signal in response to a sudden flame or explosion are well known. Such systems are available on the open market, but are high cost items. One of the reasons for this high cost has been the low sensitivity from available detectors as well as the high cost of detector manufacture. The low sensitivity results in low signal to noise ratio of the system which causes a high rate of false alarms. To circumvent the problem of false alarms, the use of individual detectors having different spectral responses has been taught by Kern, et al (see Kern et al. U.S. Pat. No. 4,296,324 entitled "DUAL SPECTRUM INFRARED FIRE SENSOR", issued Oct. 20, 1981; No. 4,691,196 entitled "DUAL SPECTRUM FREQUENCY RESPONDING FIRE SENSOR", issued Sep. 1, 1987; No. 4,769,775 entitled "MICROPROCESSOR-CONTROLLED FIRE SENSOR", issued Sep. 6, 1988; and No. 4,785,292 entitled "DUAL SPECTRUM FREQUENCY RESPONDING FIRE SENSOR", issued Nov. 15, 1988). In addition, intensity comparisons have been made between UV and IR wavelengths to further reduce false alarms. Complex microprocessor logic has been employed to analyze the flicker frequency of the radiation to distinguish a flame from background IR emission.
Axmark, et al (see Axmark et. al. U.S. Pat. No. 4,370,557 entitled "DUAL DETECTOR FLAME SENSOR" issued Jan. 25, 1983) teaches a system using dual, individual, dissimilar detectors for the control of a multi-burner boiler or industrial furnace installation. The detectors used in Axmark were a silicon (Si) detector responsive to visible light and an IR responsive lead-sulfide (PbS) detector with emphasis on the use of both the direct current (dc) and alternating current (ac) responses of these detectors.
In medical research and chemical analysis, IR spectroscopy is often useful. Instruments to perform this type of analysis typically cost $10,000.00 in 1994 US dollars.
Military applications are another expensive use of IR detection systems. Such systems are generally used for IR imaging similar to radar or for the guidance of heat seeking missiles. Although many different materials are used for these detectors, one of these is mercury-cadmium-telluride, HgCdTe, hereafter referred to as MCT. MCT detectors are cooled well below atmospheric temperatures, typically 77.degree. Kelvin, to accomplish detectivity of targets near atmospheric temperature.